298 research outputs found
Magnetic field effects on and the pseudogap onset temperature in cuprate superconductors
We study the sensitivity of and the pseudogap onset temperature,
, to low fields, , for cuprate superconductors, using a BCS-based
approach extended to arbitrary coupling. We find that and , which
are of the same superconducting origin, have very different dependences.
The small coherence length makes rather insensitive to the field.
However, the presence of the pseudogap at makes more sensitive to
. Our results for the coherence length fit well with existing
experiments. We predict that very near the insulator will rapidly
increase.Comment: 4 pages, 1 figure, contribution to the PPHMF-IV conference, Oct. 200
Magnetic Field Effects in the Pseudogap Phase: A Precursor Superconductivity Scenario
We demonstrate that the observed dependences of and on small
magnetic fields can be readily understood in a precursor superconductivity
approach to the pseudogap phase. In this approach, the presence of a pseudogap
at (but not at ) and the associated suppression of the density of
states lead to very different sensitivities to pair-breaking perturbations for
the two temperatures. Our semi-quantitative results address the puzzling
experimental observation that the coherence length is weakly dependent on
hole concentration throughout most of the phase diagram. We present our
results in a form which can be compared with the recent experiments of
Shibauchi et al, and argue that orbital effects contribute in an important way
to the dependence of .Comment: 6 pages, 1 figure, elsart.cls included. Submitted to the proceeding
of SNS 2001, Chicag
Auntie knows best? public broadcasters and current affairs knowledge
Public service broadcasters (PSBs) are a central part of national news media landscapes. In many countries, PSBs are the first choice of citizens when it comes to news providers. And in perhaps more countries still, PSBs are thought of as specialists in provision of hard news. We test this proposition here using survey data from a large crossnational survey involving indicators of current affairs knowledge and media consumption. Specifically, we examine whether exposure to public versus commercial news influences the knowledge citizens possess about current affairs, both domestically and internationally. We
also test, using propensity score analysis, whether there is variation across PSBs in this regard. Results indicate that compared to commercial news, watching PSB has a net
positive influence on knowledge of hard news, though not all PSBs are equally effective in contributing to knowledge acquisition. This knowledge gap between PSB and commercial
news media consumption appears to be mitigated by factors such as de jure independence,proportion of public financing, and audience share
The pseudogap state in superconductors: Extended Hartree approach to time-dependent Ginzburg-Landau Theory
It is well known that conventional pairing fluctuation theory at the Hartree
level leads to a normal state pseudogap in the fermionic spectrum. Our goal is
to extend this Hartree approximated scheme to arrive at a generalized mean
field theory of pseudogapped superconductors for all temperatures . While an
equivalent approach to the pseudogap has been derived elsewhere using a more
formal Green's function decoupling scheme, in this paper we re-interpret this
mean field theory and BCS theory as well, and demonstrate how they naturally
relate to ideal Bose gas condensation. Here we recast the Hartree approximated
Ginzburg-Landau self consistent equations in a T-matrix form. This recasting
makes it possible to consider arbitrarily strong attractive coupling, where
bosonic degrees of freedom appear at considerably above . The
implications for transport both above and below are discussed. Below
we find two types of contributions. Those associated with fermionic
excitations have the usual BCS functional form. That they depend on the
magnitude of the excitation gap, nevertheless, leads to rather atypical
transport properties in the strong coupling limit, where this gap (as distinct
from the order parameter) is virtually -independent. In addition, there are
bosonic terms arising from non-condensed pairs whose transport properties are
shown here to be reasonably well described by an effective time-dependent
Ginzburg-Landau theory.Comment: 14 pages, 5 figures, REVTeX4, submitted to PRB; clarification of the
diagrammatic technique added, one figure update
Origin of the pseudogap phase: Precursor superconductivity versus a competing energy gap scenario
In the last few years evidence has been accumulating that there are a
multiplicity of energy scales which characterize superconductivity in the
underdoped cuprates. In contrast to the situation in BCS superconductors, the
phase coherence temperature Tc is different from the energy gap onset
temperature T*. In addition, thermodynamic and tunneling spectroscopies have
led to the inference that the order parameter is to be
distinguished from the excitation gap ; in this way, pseudogap effects
persist below Tc. It has been argued by many in the community that the presence
of these distinct energy scales demonstrates that the pseudogap is unrelated to
superconductivity. In this paper we show that this inference is incorrect. We
demonstrate that the difference between the order parameter and excitation gap
and the contrasting dependences of T* and Tc on hole concentration and
magnetic field follow from a natural generalization of BCS theory. This
simple generalized form is based on a BCS-like ground state, but with self
consistently determined chemical potential in the presence of arbitrary
attractive coupling . We have applied this mean field theory with some
success to tunneling, transport, thermodynamics and magnetic field effects. We
contrast the present approach with the phase fluctuation scenario and discuss
key features which might distinguish our precursor superconductivity picture
from that involving a competing order parameter.Comment: 4 pages, 2 EPS figures, use LaTeX package espcrc2.sty from Elsevier,
submitted to SNS'01 conference proceeding
Degenerate recognition of MHC class I molecules with Bw4 and Bw6 motifs by a killer cell Ig-like receptor 3DL expressed by macaque NK cells
The killer cell immunoglobulin-like receptors (KIRs) expressed on the surface of natural killer (NK) cells recognize specific major histocompatibility complex class I (MHC-I) molecules and regulate NK cell activities against pathogen-infected cells and neoplasia. In human immunodeficiency virus (HIV) infection, survival is linked to host KIR and MHC-I genotypes. In the simian immunodeficiency virus (SIV) macaque model, however, the role of NK cells is unclear due to the lack of information on KIR-MHC interactions. Here, we describe the first characterization of a KIR-MHC interaction in pig-tailed macaques (Macaca nemestrina). Initially, we identified three distinct subsets of macaque NK cells that stained ex vivo with macaque MHC-I tetramers loaded with SIV peptides. We then cloned cDNAs corresponding to 15 distinct KIR3D alleles. One of these, KIR049-4, was an inhibitory KIR3DL that bound MHC-I tetramers and prevented activation, degranulation and cytokine production by macaque NK cells after engagement with specific MHC-I molecules on the surface of target cells. Furthermore, KIR049-4 recognized a broad range of MHC-I molecules carrying not only the Bw4 motif but also Bw6 and non-Bw4/Bw6 motifs. This degenerate, yet peptide-dependent, MHC reactivity differs markedly from the fine specificity of human KIRs
Magnetic Field Effects in the Pseudogap Phase: A Competing Energy Gap Scenario for Precursor Superconductivity
We study the sensitivity of T_c and T^* to low fields, H, within the
pseudogap state using a BCS-based approach extended to arbitrary coupling. We
find that T^* and T_c, which are of the same superconducting origin, have very
different H dependences. This is due to the pseudogap, \Delta_{pg}, which is
present at the latter, but not former temperature. Our results for the
coherence length \xi fit well with existing experiments.We predict that very
near the insulator \xi will rapidly increase.Comment: 4 pages, 4 figures, RevTe
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